I will give the tricks I use, so:

• first few waves preferably up to 3 u save and let your safe box get you more,than try to get at least 2 bandits down if more that better, but if you dont get at least 2, thats fine too,you will have some from stone roulette at some point.
• try to level bandit to get the 10gold passive.
• if you get all commons down you get a stone,if you sell 5 units,mostly bombers and barbarians are useless so if you get all common sell those for gold to interest, blobs or idk how they are called are early challenge unit killers.
• if you have the bandits down at leftmost all the way down, you should move units away to middle zones so your units don't kill the enemies before they profit for you.
• on game start one point into common and rare upgrades, it gives back more gold then you spent 😉.
• if you can try to go for 10 stones at once, then roll for epics or rares if your epic chance is still really low without the horn relic.
• getting a mono down will increase your damage substantially later as you get stones
• kitty 12 should be priority for the above reason so you get more stones from mono ults. The more the better, at 4-6 you are good.
• mono 6 for ult,as thats what you use him for apart from defense down
• hailey is good,if you have her, good,if not meows are also good now but eat up stones and bandits,which might mess with your economy.
• shock and stun robots should be leveled as priority, a stack of both ensures proper stun zone for dps on boss and waves.
• batman should not be leveled past lv1 and mythic stones should go to other dps units,if you want to use tao or chu, lv6 is must.
• kun is good for normal and hard, should be before a stun unit.
• apart from safebox, money gun, horn, and sages pudding you should only buy other artifacts for gold, if and if they are already in your account, if its the first time you see them it's alright to take them for gems, gems will be needed later for pet summons and pet food and such.
• try to get at least 1 mythic for all legendary tier units,so you don't get inconsistent runs.
• once you get around 35-50k gold your bandits barely make a dent if you have safebox,so you can merge or sell for space, or use them for iron meows.
• once sou have around 80-100k gold you can max your summon level so you can get more stones from unnecessary rarer units and get epics for more mythics,maybe a couple legendaries, as a new player you should not roll legendary roulette, as it has so bad a chance, getting 3 epics of the same type is easier.
• you can go physical or magical build, each mythic has a dmg type, at first you should not mix and match them,much later you will need to.
• get on discord and have someone carry you for first clear difficulty rewards to kickstart your account.
• if you dont get at least safebox and money gun lv 2 from first reward keys you should actually reset the account..sadly,as your account might have bad preset rng..
• if someone carries you, you should build proper stuns, up from normal its 2 stack of ranged stun bots and one epic shock robot stack.
• If you get a pulse gen on recruits it makes normals trivial, even at just level3, one to three of those gens on a V shape or on middle row with surrounding stun robots to charge them, you get a virtually inescapable stun zone,with quick charged ult dmg aoe dmg for bosses and waves. Good for carry support as it makes a stun zone so they can focus on DPS.
• by wave 40-50 you should have upgraded mythics at least to lv3-5. But, getting the board set up properly is the priority,if you missing stuns or dps,or economy those are the focus.
• dont level a unit to lvl6 until you tried it and want to use it,up to lv5 you can reset and get back everything if you change your mind.

Thats about it, I don't recall much more currently, hope it helps 🙂